The staff have said that they won't help you get your pets back if they are "stolen" - loaned to somebody, but not returned. While I get that the staff don't want to go digging through records to find the evidence of sending or not, I want to be sure that lying like this is discouraged. So, here's what I propose. A board called "Contracts" or something like that where a person can write a contract (something simple like "I am loaning x pet to x user. That user will send the pet back at the agreed upon time"). Then, the other person posts with "I agree" and the first user takes a screenshot, then sends the pet. This way, if the pet is not returned, the screenshot can be sent to the staff, who will ask the user to give back your pet, or take it back. This is just an idea, so please comment, like and dislike.

Automated item-triggered contracts, perhaps? (A craftable scroll, book, etc...Maybe make cuttlefish ink useful?)

Perhaps there could be a lend pet feature similar to gifting/selling where there is a timer associated with the trade. Once the timer expires the pet will automatically be transferred back to the original owner's inventory. Perhaps it, like the sell pet option, could have an optional cash amount field for when pets are rented out for a cash fee.

Otherwise, if the staff refuses to persue it, it then falls to the players to instill their own method into this potential problem. Making the thefts and other misdeeds public, for instance, is a very easy way for people to get blacklisted from people ever being able to do business with other players.

Other than that, the only method I can think of is common sense - only lend to people you can trust, or never trade pets you would miss if someone took off with them.